DISCOVERY OF THE LAWS OF THE COLOURS OF THIN PLATES.
THE facts which we have now to consider are remarkable, inasmuch as
the colours are produced merely by the smallness of dimensions of
the bodies employed. The light is not analysed by any peculiar {77}
property of the substances, but dissected by the minuteness of their
parts. On this account, these phenomena give very important
indications of the real structure of light; and at an early period,
suggested views which are, in a great measure, just.
Hooke appears to be the first person who made any progress in
discovering the laws of the colors of thin plates. In his
_Micrographia_, printed by the Royal Society in 1664, he describes,
in a detailed and systematic manner, several phenomena of this kind,
which he calls "fantastical colors." He examined them in _Muscovy
glass_ or mica, a transparent mineral which is capable of being
split into the exceedingly thin films which are requisite for such
colors; he noticed them also in the fissures of the same substance,
in bubbles blown of water, rosin, gum, glass; in the films on the
surface of tempered steel; between two plane pieces of glass; and in
other cases. He perceived also,[48\9] that the production of each
color required a plate of determinate thickness, and he employed
this circumstance as one of the grounds of his theory of light.
[Note 48\9: _Micrographia_, p. 53.]
Newton took up the subject where Hooke had left it; and followed it
out with his accustomed skill and clearness, in his _Discourse on
Light and Colors_, communicated to the Royal Society in 1675. He
determined, what Hooke had not ascertained, the thickness of the
film which was requisite for the production of each color; and in
this way explained, in a complete and admirable manner, the colored
rings which occur when two lenses are pressed together, and the
_scale of color_ which the rings follow; a step of the more
consequence, as the same scale occurs in many other optical
phenomena.
It is not our business here to state the hypothesis with regard to
the properties of light which Newton founded on these facts;--the
"fits of easy transmission and reflection." We shall see hereafter
that his attempted induction was imperfect; and his endeavor to
account, by means of the laws of thin plates, for the colors of
natural bodies, is altogether unsatisfactory. But notwithstanding
these failures in the speculations on this subject, he did make in
it some very important steps; for he clearly ascertained that when
the thickness of the plate was about 1⁄178000th of an inch, or three
times, five times, seven times that magnitude, there was a bright
color produced; but blackness, when the thickness was exactly
intermediate between those magnitudes. He found, also, that the
thicknesses which gave red and {78} violet[49\9] were as fourteen to
nine; and the intermediate colors of course corresponded to
intermediate thicknesses, and therefore, in his apparatus,
consisting of two lenses pressed together, appeared as rings of
intermediate sizes. His mode of confirming the rule, by throwing
upon this apparatus differently colored homogeneous light, is
striking and elegant. "It was very pleasant," he says, "to see the
rings gradually swell and contract as the color of the light was
changed."
[Note 49\9: _Opticks_, p. 184.]
It is not necessary to enter further into the detail of these
phenomena, or to notice the rings seen by transmission, and other
circumstances. The important step made by Newton in this matter was,
the showing that the rays of light, in these experiments, as they
pass onwards go periodically through certain cycles of modification,
each period occupying nearly the small fraction of an inch mentioned
above; and this interval being different for different colors.
Although Newton did not correctly disentangle the conditions under
which this periodical character is manifestly disclosed, the
discovery that, under some circumstances, such a periodical
character does exist, was likely to influence, and did influence,
materially and beneficially, the subsequent progress of Optics
towards a connected theory.
We must now trace this progress; but before we proceed to this task,
we will briefly notice a number of optical phenomena which had been
collected, and which waited for the touch of sound theory to
introduce among them that rule and order which mere observation had
sought for in vain.